The present invention relates to an air pressure monitoring system that wirelessly transmits information regarding tire air pressure.
Wireless tire air pressure monitoring systems for allowing a driver in a vehicle passenger compartment to check the pressure of vehicle tires have been proposed. One such monitoring system includes transmitters and a receiver. Each transmitter is located in one of the wheels and the receiver is located in the body frame of the vehicle. Each transmitter detects the pressure of the associated tire and wirelessly transmits the detection information. The receiver receives information from the transmitters and displays the information regarding the pressure of each tire on, for example, a display located in front of the driver's seat.
The transmitters are located in the tires. Each transmitters transmits information regarding the pressure of the associated tire to the common receiver. The receiver must distinguish signals from the tires. Therefore, a transmitter having its own specific ID data, or ID code, has been proposed. The transmitter transmits air pressure data with its own ID code attached, which permits the receiver to identify the transmitter based on the ID codes.
In a typical monitoring system, each transmitter transmits detection data to a receiver at predetermined intervals. The intervals are previously programmed in a microcomputer in each transmitter.
The time intervals stored in each microcomputer have the same fixed value. Therefore, all the transmitters transmit signals at the same time intervals, which increases possibility that two or more transmitters transmit signals at the same time. In this case, the signals interfere with one another, and the receiver cannot properly receive the signals.
A typical automobile has four transmitters, each of which is provided in one of the tires. It has been confirmed that there is a 0.017% chance of interference of signals if each transmitter transmits signals every ten minutes and the duration of each transmission is 192 msec.
As described above, intervals of signal transmissions are fixed and the same for each microcomputer. Thus, once two or more transmitters have the same signal transmission timing, the matched timing will be maintained for relatively long period. Accordingly, the receiver cannot properly receive signals for relatively long time.
To solve this problem, Japanese Unexamined Patent Publication No. 8-505939 discloses a system for differentiating transmission intervals, which are programmed in the microcomputers of the transmitters. This method reduces the chance of coincidence of signal transmissions and prevents matched timings from being maintained. However, programming different interval data to each transmitter is troublesome and is not suitable for mass production of transmitters.
Further, it is impossible to differentiate transmission timings of all the transmitters, which are mass-manufactured. Therefore, two or more transmitters having the same transmission timing may be mounted on one automobile.
ID codes, which are allotted to each transmitter, are useful to identify the transmitters. However, the receiver must be able to distinguish a different ID code from each transmitter. The apparatus of Publication No. 8-505939 executes a registration process for relating the ID code of each transmitter to the position of the associated tire. Specifically, when manufacturing an automobile, the transmitters are actuated by putting a magnet near the tires. The actuated transmitters transmit learning signals, which include an ID code, to the receiver. The ID code of each transmitter is registered in the receiver.
However, the system of Publication No. 8-505939 requires a person to perform the registration process and a special programming tool, or magnet. Therefore, registration of ID codes in the receiver is troublesome. Further, the registration process needs to be performed after a service to the vehicle, such as a change of tires. This complicates the maintenance of the vehicle.
Japanese Unexamined Patent Publication No. 9-210827 discloses a system for automatically renewing ID codes of transmitters registered in a receiver. Specifically, the receiver receives ID codes from four transmitters attached to four tires of a vehicle and stores the received ID codes as main ID codes. When receiving ID codes other than the main ID codes, the receiver stores the received ID codes as backup codes. When the receiver cannot receive one of the main ID code, the receiver deletes that main ID code, or inactive ID code. The receiver then selects an active ID code, which is constantly received, among the backup ID codes and renews the deleted ID code with selected backup ID code.
The system of Publication No. 9-210827 judges that a certain main ID code is inactive when the main ID code has not been received in a relatively short period. Therefore, even if there is no lasting malfunction in a transmitter, for example, if the ID code of the transmitter is not received by the receiver due to an external cause such as fading, the system judges that the main ID code is inactive. The system then replaces the main ID code with a backup ID code, which is actually unnecessary, which hinders proper communication between the transmitters and the receiver.